Graft copolymers formed by polymerizing monomers at active grafting sites on a polymer backbone constitute an interesting class of polymer hybrids because, although a single chemical species, they nevertheless are capable of displaying properties characteristic of the graft polymer as well as of the backbone polymer, rather than a simple averaging of the properties of the component polymers. When compared to physical blends of polymers, graft copolymers, owing to the intersegment chemical bonds therein, usually exhibit a finer heterophasic morphology in which the domain size of the dispersed phase is stable and may be smaller by about an order of magnitude. Moreover, the adhesion between phases is better. Physical blends of immiscible polymers, e.g., polypropylene and polystyrene, require the inclusion of a compatibilizer, e.g., a block copolymer suitably chosen, which can alleviate to some degree the problem associated with the high interfacial tension and poor adhesion between the immiscible polymers in the blend. Physical blends of polypropylene and polystyrene containing minor amounts of a styrenic block copolymer rubber as a compatibilizer for the polymers in the blend are described in U.S. Pat. No. 4,386,187.
Structural plastics based on a "chemical" blend of propylene and styrene polymers, i.e., based on a graft copolymer of a styrene polymer on a backbone of propylene polymer material, would fill a need in the art because of the benefits accruing from the fine domain structure in the graft copolymers and also because the necessary adhesion between the propylene polymer and styrene polymer phases would derive from the chemical bonds in the graft copolymer per se rather than depend on the action of an external agent, i.e., a compatibilizer. However, until now, the uses suggested for these graft copolymers have been limited chiefly to compatibilizers for immiscible polymer systems and components of gum plastic compositions.
U.S. Pat. No. 3,314,904 describes forming a "gum plastic" by making a graft copolymer of styrene on polyethylene or polypropylene, and, in particular, a graft interpolymer of styrene, acrylonitrile, and polyethylene or polypropylene, and blending the graft copolymer with certain selected compatible rubbery materials. The grafted styrene or styrene-acrylonitrile content of the graft copolymer is 75-95%, preferably 85-95%, and more preferably 90-95%. Hence the graft copolymer is predominantly bound styrene or bound styrene-acrylonitrile, and in the copolymers made from polypropylene the polypropylene is only a minor component and present as a dispersed phase. Thus the properties of the bound styrene or styreneacrylonitrile predominate. The graft copolymer is made by subjecting the polyolefin to high-energy ionizing radiation, and then contacting the irradiated polyolefin with styrene or with styrene and acrylonitrile.
However, for use as stand-alone structural plastics having the desirable properties of propylene polymers, e.g., excellent chemical resistance, good moisture resistance, etc., graft copolymers of a styrene polymer on a backbone of a propylene polymer material must exhibit a heterophasic morphology in which the propylene polymer is the continuous phase. This requires that the styrene polymer content of the graft copolymer not exceed about 65 percent by weight, while, at the same time, being high enough to improve the stiffness of the propylene polymer to the required degree.
The advantages of graft copolymers of a styrene polymer on a propylene polymer backbone over physical blends of the polymers as stand-alone structural plastics could be better utilized if a means were found for imparting a better balance of properties to the graft copolymers.